Five years ago, Winfried Kretschmann caused a political sensation in Germany when the now 67-year-old became the nation's first and only member of the Green Party to become a government leader.

Key points:

13 million people in three German states voting on Sunday

Opinion polls show Greens ahead of Merkel's Christian Democrats with 33.5 per cent of vote

History shows Baden-Wuerttemberg voters prefer the Greens

This Sunday, when voters in the conservative south-western state of Baden-Wuerttemberg go to the polls, Mr Kretschmann could set off a political earthquake if his party emerges for the first time as the biggest political bloc in one of the nation's parliaments.

Opinion polls showed the Greens under the popular Mr Kretschmann — a former biology teacher — edging ahead of Chancellor Angela Merkel's conservative Christian Democrats (CDU), with 33.5 per cent of the vote.

CDU support has slumped to 28.5 per cent from the previous 39 per cent at the 2011 election, when Mr Kretschmann and the left-leaning Social Democrats (SPD) formed a coalition ending the CDU's six-decade grip on Baden-Wuerttemberg.

About 13 million voters will go to the polls on Sunday in three state elections, with neighbouring Rhineland-Palatinate and Saxony-Anhalt in the east also set to elect new governments.

But the election in Baden-Wuerttemberg could also have consequences beyond the state.

Success for Mr Kretschmann could help Ms Merkel forge a new political centre in Germany, as the refugee crisis drives right-wing voters away from her CDU party, and into the arms of a new populist anti-foreigner party, the Alternative for Germany.

Kretschmann's conservative views at odds with party

Since taking office five years ago, Mr Kretschmann has transformed himself into a business-friendly leader of Baden-Wuerttemberg, which is home to some of Germany's biggest brand names including Mercedes Benz, Porsche, Hugo Boss and computer giant SAP.

"This is a state with strong traditions and ... I live those traditions" Mr Kretschmann, the former student Maoist, has said.

A practising Catholic, Mr Kretschmann has cast himself as father of the state with his pragmatic conservative politics frequently placing him at odds with other more left-wing Green members.

"Five years of government in Baden-Wuerttemberg has left me a frustrated party member," Joerge Rupp, a member of the Greens left wing, said.

Still, Mr Kretschmann is optimistic about securing victory on the weekend after the anti-nuclear Greens rode to power in 2011 on the back of concerns about atomic energy triggered by the Fukushima nuclear disaster.

Voters in Baden-Wuerttemberg also swung behind the Greens in 2011 after they opposed the former CDU-led state government's unpopular plans to build a new central railway in the state capital, Stuttgart.

But while the CDU might be facing an electoral debacle in Baden-Wuerttemberg, Sunday's vote could open up new national coalition options for Ms Merkel, especially as her present coalition partner, the SPD, could be the big loser on the weekend.

Mr Kretschmann has ruled out a move to the national political stage, but has praised Ms Merkel's crisis management skills and her handling of the refugee crisis, referring to her as "our chancellor".

Polls show that more than 60 per cent of Green Party voters also support Ms Merkel's liberal refugee stance.

This week, Ms Merkel took the unusual step at a Baden-Wuerttemberg campaign rally of telling conservative voters if they wanted to back her they should vote CDU and not the Greens.

Mr Kretschmann is however just the latest political manifestation of Germany's Greens, which first surged into the national parliament in 1983 on a pro-environment and pacifist platform.

Despite conflicts between the Greens' so-called fundamentalist and realist wings, the party played a major role in driving welfare and labour reforms as the junior member of the SDP-led government of former chancellor Gerhard Schroeder, which Ms Merkel defeated in 2005.

But the Greens lurched to the left at the last national elections in 2013, calling for a tax on the rich. This was despite the party having some of Germany's highest-income earners among its supporters.

The result was a hefty 2.3 per cent swing against the party.

The SPD's political decline combined with the Greens' disastrous 2013 election campaign forced the Greens to also seek out new coalition partners.

A few months after the national election, the Greens joined the CDU-led coalition in the state of Hesse, which borders Baden-Wuerttemberg.

The Greens' decision to team up with the CDU in Hesse under a close ally of Ms Merkel, Volker Bouffier, could now be the model for a possible national CDU-Greens coalition after the 2017 election.

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